qemu: report an error if memballoon has wrong address type
qemu: report an error if memballoon has wrong address type

qemuBuildMemballoonDevStr returns NULL if memballoon doesn't have
the right address type, but it doesn't report an error, leading to:
error: An error occurred, but the cause is unknown

Report a helpful error message instead, e.g.:
error: XML error: memballoon unsupported with address type 'usb'

crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,datto/libvirt,eskultety/libvirt,shugaoye/libvirt,elmarco/libvirt,fabianfreyer/libvirt,libvirt/libvirt,VenkatDatta/libvirt,agx/libvirt,rlaager/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,elmarco/libvirt,libvirt/libvirt,agx/libvirt,agx/libvirt,jfehlig/libvirt,zippy2/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,crobinso/libvirt,taget/libvirt,cbosdo/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,crobinso/libvirt,libvirt/libvirt,shugaoye/libvirt,nertpinx/libvirt,rlaager/libvirt,fabianfreyer/libvirt,datto/libvirt,VenkatDatta/libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,nertpinx/libvirt,eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,shugaoye/libvirt,crobinso/libvirt,taget/libvirt,nertpinx/libvirt,taget/libvirt,datto/libvirt,libvirt/libvirt,cbosdo/libvirt,elmarco/libvirt,datto/libvirt,andreabolognani/libvirt,nertpinx/libvirt,taget/libvirt,jfehlig/libvirt,rlaager/libvirt,rlaager/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,andreabolognani/libvirt,taget/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,jfehlig/libvirt,andreabolognani/libvirt,elmarco/libvirt,datto/libvirt,jardasgit/libvirt,shugaoye/libvirt,eskultety/libvirt,elmarco/libvirt,cbosdo/libvirt,jardasgit/libvirt,agx/libvirt,cbosdo/libvirt,cbosdo/libvirt,rlaager/libvirt,agx/libvirt,olafhering/libvirt,olafhering/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,jardasgit/libvirt